It is common to back up a real-time clock (RTC) with a battery, such as a long-life lithium battery, so that when a main power supply for the RTC fails, or has been disconnected, the RTC will continue to indicate the correct time and provide correct time intervals, such as for periodically recording data. It is also known to provide voltage sensing battery degradation or failure arrangements, to indicate when a battery should be replaced.
In many systems which use a RTC, an analog input may not be available for monitoring the voltage of a back-up battery, or it may not be desirable to dedicate an analog input to such a purpose. It is thus desirable, and it is an object of the invention, to determine when a back-up battery for a RTC should be replaced, without directly monitoring the output voltage of the back-up battery.